Nowadays, bio-implantable medical devices are used to treat various diseases. Some of the bio-implantable medical devices are based on neurostimulation as a therapeutic principle and can be used for the treatment and management of diseases, such as pain, constipation, sleep disturbances, Parkinson's disease, epilepsy, hand tremor, and myodystonia. Representative examples of such neurostimulation devices include spinal cord stimulation (SCS) systems and deep brain stimulation (DBS) systems.
FIG. 1 is a schematic diagram of a deep brain stimulation system and shows a state in which a neurostimulation electrode applies a neural stimulus to the thalamus. A neurostimulation-based bio-implantable medical device, such as an SCS or DBS device, essentially includes a neurostimulator adapted to generate pulse signals for electrical neurostimulation and at least one electrode inserted into a nerve site in need of treatment and through which the neurostimulation signals are applied to the nerve site (see FIG. 1). The neurostimulation electrode of the DBS device exemplified in FIG. 1 penetrates the cranium but it is to be understood that the neurostimulation electrode may also be inserted or implanted into any specific nerve site of the spinal cord, the peripheral nervous system or the brain. For example, a bio-implantable electrode of an SCS device may be inserted so as to penetrate the greater pectoral muscle, abdomen or buttock of a patient. In the neurostimulation-based bio-implantable device, the at least one electrode is usually connected to a long lead, as shown in FIG. 1. The lead is responsible for the electrical connection between the electrode and the neurostimulator. The electrode of the device is often inserted into the subcutaneous tissue layer of the patient. In many practical cases, however, the nerve coming into contact with the electrode is positioned deeper in the body. Thus, a connector is provided to connect the neurostimulator to the lead. In many cases, the neurostimulator and the connector are positioned outside the body.
Such an implantable or insertable electrode is used for neurostimulation but should not be reduced in size below a predetermined limit in order to effectively conduct electricity. Accordingly, the electrode cannot be inserted into the body without a relatively extensive surgical incision. For example, DBS treatment requires an extensive incision of the scalp and cranium of a patient. This incision should be made under general anesthesia and causes great inconvenience for the patient.
Highly elastic metals and alloys, such as gold, stainless steel, tungsten, and platinum-iridium alloys, are currently used to produce electrodes for neurostimulation but they are not satisfactory in bioaffinity. When such a highly elastic metal or alloy is used as an electrode material, it tends to react with body fluids. This reaction causes the corrosion of the electrode material in the body, which leads to the limited service life of the electrode. Also, the patient may suffer from pain and inflammation due to the corroded electrode.
Due to its relatively large size, the electrode delivers stimuli to a wide range of sites as well as a target site in the cerebral cortex and should apply strong stimuli to achieve a desired level of stimulation. Further, the production of the electrode requires the construction of a mold and process variations depending on the size and shape of the electrode, which are rather troublesome problems.
Moreover, a combination of neurostimulation therapy and diagnosis by magnetic resonance imaging (MRI) is being increasingly used to observe the therapeutic effect of neurostimulation. MRI is a noninvasive diagnostic tool that provides high resolution 3-dimensional images. However, patients are exposed to a rather strong electromagnetic field during MRI. The intensity of the electromagnetic field increases with increasing resolution of the MRI system. The MRI system emits a static magnetic field, a gradient magnetic field, and a radio-frequency magnetic field, which is generated from the transmitter of the scanner. That is, the MRI system emits a total of three magnetic field components. The static magnetic field has an intensity of about 0.2 to about 3.0 Tesla (T). 3.0 T corresponds to about 60,000 times greater than the earth's magnetic field. The intensity of the gradient magnetic field varies with time. 0 to 5 kHz can produce a gradient magnetic field of 40 mT/m. The radio frequency magnetic field is large in scale and can output a maximum energy of 20,000 W (corresponding to 20 times or more that of a toaster) with 64 MHz at a static magnetic field of 1.5 T. The magnetic fields of the MRI system adversely affect a neurostimulation device. For example, the static magnetic field applies a force to the neurostimulation device placed in the scanner of the MRI system. As a result, the neurostimulation device may be stressed. The radio-frequency magnetic field heats the metal parts of the neurostimulation device. Further, the time-dependent gradient magnetic field gives rise to an induced current, causing damage to the neurostimulation device and imaging disturbances. The most serious negative effect of the magnetic fields is that the lead of the neurostimulation device may be overheated by the radio-frequency magnetic field. Some research results reveal that the temperature of the lead is raised to 25° C. or above. The negative effects of the magnetic field of MRI systems on neurostimulation devices can be found, for example, in Rezai et al., Journal of Magnetic Resonance Imaging, Vol. 15(2002), pp. 241-250.
There is a need in the art for electrode materials that can be safely operated in the magnetic fields of an MRI system, have excellent mechanical properties, are sufficiently thin, and can replace metal materials, but the need is still not met.